The present invention pertains to an optical gas sensor with at least one radiation source, with a reference detector and with at least one measuring detector in a reflecting measuring gas cuvette.
Compact gas analyzers can make possible low manufacturing costs and a robust design because no movable optical components are used. These may be made available with such gas sensors, as disclosed, e.g., in DE 195 20 488 C1 and in U.S. Pat. No. 5,973,326, which form this gas analyzer type.
The known principle of measurement of the gas sensors of this type is based on the concentration-dependent absorption of electromagnetic radiation especially in the infrared wavelength range by the gas to be measured, the measuring gas. The measuring gas, e.g., hydrocarbons, CO2 and other trace gases, diffuses, in general, through a dust protection membrane or a flame arrester in the form of a fabric or a gas-permeable layer of a sintered or ceramic material into the cuvette volume of the measuring gas cuvette of the gas sensor.
The radiation of at least one broad-band radiation source covering, in general, a rather broad wavelength range passes through the measuring gas cuvette. An incandescent lamp or an electrically heated glass or ceramic element is usually used as the radiation source. The radiation emitted divergently from the electromagnetic radiation source, of which there is at least one, is bundled by means of optically reflecting surfaces in order to increase the radiation intensity at the site of the measuring and reference detectors. The signal-to-noise ratio of the gas sensor is increased by the bundling of the radiation and the quality of the measurement is thus improved. The detectors used are, in general, pyroelectric crystals, semiconductor elements or so-called thermoelectric piles formed of thermocouples. These different types of detectors convert radiation power into electric signals. The signals are evaluated in a suitable manner for the determination of the gas concentration to be measured. If two or more different measuring gases are to be measured with one gas sensor, a number of measuring detectors, whose number corresponds to the number of the different measuring gases, are used, which are sensitive to a particular measuring gas in a wavelength-specific manner. The wavelength range or wavelength ranges is/are selected by means of interference filters, which are connected, in general, directly to the corresponding detectors or are combined with same. A first wavelength range contains the wavelength of an absorption band of the measuring gas and is detected by the corresponding measuring detector, while the second wavelength range detected by the reference detector is selected such that it is not affected by the absorption of the measuring gas. The concentration of the measuring gas is determined and the influence of aging effects of the radiation source as well as the effect of possible contaminations in the optical beam path are compensated by forming the quotient and by suitably taking into account the measured signals.
The object of the present invention is to provide a gas sensor of this type, which makes possible a very compact design with improved measuring sensitivity.
According to the invention, an optical gas sensor is provided with at least one radiation source, with a reference detector and with at least one measuring detector in a reflecting measuring gas cuvette. The measuring gas cuvette has a first inner cylinder section and a second outer cylinder section defining an annular space therebetween. The first inner cylinder section and the second outer cylinder section are preferably concentric. The annular space is limited by a cover element and a bottom element arranged at a spaced location therefrom in the direction of a longitudinal axis. The cover element is permeable to the measuring gas. The bottom element accommodates the radiation source.
The measuring detector, of which there is at least one, and/or the reference detector may be arranged in the bottom element. The cover element and the bottom element may extend essentially in parallel to one another and at right angles to the central longitudinal axis of the cylinder sections. A temperature sensor may be arranged in the first, inner cylinder section.
The radiation source and one of the measuring detectors are arranged in opposite sections of the annular space.
The radiation source and the measuring detector may be arranged at closely spaced locations next to one another with a reflecting wall blocking the annular space in the circumferential direction, so that direct beam paths from the radiation source to the measuring detector are blocked in the circumferential direction in the annular space and the radiation travels circumferentially opposite the direct path around the first, inner cylinder section.
One important advantage of the gas sensor according to the present invention is the rotationally symmetrical design of the measuring gas cuvette as an annular space, as a result of which the beam path or beam paths between the radiation source or radiation sources and the detectors is made longer, on average, because of multiple reflections, and as a result of which simple manufacture without complicated adjustments is also possible.
Furthermore, it is advantageous that the gas-carrying volume is reduced by the first, inner cylinder section, so that a shorter response time of the gas sensor according to the present invention is obtained as a result.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.